Effect of heavy metal stress in plant metabolism of solanaceous plant species with emphasis on nitrogen assimilation

Abstract

Several plant species are able to accumulate and withstand large quantities of heavy metals in their tissues without dramatic alterations in their growth usually observed in plants. Such metal accumulating plants are tested and used for remediation of contaminated soils and waters. Although the literature provides extensive information on the effect of heavy metals in growth and development of several metal-accumulating and non accumulating plants, nitrogen metabolism and the regulation of related enzymes have not been widely studied. In an effort to better understand the responses of plants species under heavy metal stress, a comparative study was held between two solanaceous species, Nicotiana tabacum and Nicotiana glauca, plants with promising properties for phytoremediation. Plants of the two species were grown in the presence of different concentrations of the heavy metals zinc (Zn), nickel (Ni) and cadmium (Cd) in the following experimental systems in the greenhouse: in pots containing artificial substrate for 6 weeks and hydroponically for 8 days. In order to get a global picture for the heavy metal –induced variations concerned the examined species, the accumulation and distribution of metals in various plants tissues (lower and upper leaves, lower and upper shoots, roots) was determined. Also, several morphological parameters related with plants growth, physiological related with light and dark reactions of photosynthesis and water balance, biochemical related with nitrogenous compounds metabolism and the enzymes involved in ammonia assimilation, were followed. The presence of the referenced metals in the culture medium induced alterations, in all the parameters examined. In particular, strong positive correlations exhibited between the accumulation of Zn, Ni and Cd in the examined tissues and the supplied metals concnentrations. Our data revealed different accumulation patterns for each examined metal at tissue and species level. Furthermore, both growth and physiological parameters were negatively affected at least by the higher concentration of the three metals tested. However, differential responses were observed between N. tabacum and N. glauca, where the latter seemed to be more sensitive at the higher concentration of the metals, especially for Ni and Cd, showing heavier symptoms. In addition, proline accumulation was positively affected over the range of the three metals concentrations. Alterationsinduced in quantitive and qualitive profile of soluble proteins corresponding to Zn, Ni and Cd treatments. Furthermore, the concentration of Zn, Ni and Cd differentially affected the specific activities and the protein levels of ammonia assimilating enzymes glutamine synthetase (GS), glutamate synthase (Fd-GOGAT) and glutamate dehydrogenase (GDH).